To be suitable for sealing tire punctures, a sealant composition must meet a unique and exceptionally demanding set of physical and chemical criteria. It must be resistant to aging, decomposition and flow at the high temperatures to which tires are heated under summertime driving conditions. In the case where the puncturing object remains in the tread while the tire continues to be used, the sealant must have sufficient tack and fatigue resistance to remain adhered to the object even as it works back and forth during tire revolution. In the case where the puncturing object is removed from the tread, the sealant must be capable of flowing into the puncture hole at wintertime temperatures and sealing it. Further properties required of a tire sealant are discussed in greater detail below.
Because butyl rubber has low air permeability, high resistance to aging and an easily controlled cross-linked density, the prior art has attempted to utilize butyl rubber as a basic compound of tire sealants. One approach, exemplified by U.S. Pat. Nos. 2,756,801, 2,765,018 and 2,782,829, has been to utilize a single grade of butyl rubber to form the sealant network, and to add tackifiers, plasticizers and other more specialized ingredients such as phenols or iron ozide in an attempt to achieve the necessary balance of physical properties. Compositions based on such patents, however, have not achieved widespread acceptance, primarily because such sealants have failed to perform satisfactorily at the temperature extremes (e.g. -20.degree. F. to 220.degree. F.) to which tires are subjected.
A second approach to butyl rubber based tire sealants has utilized a combination of high and low molecular weight butyl rubber grades cross-linked together to form a single elastomeric network. Such sealants have been found to perform quite well over wide temperature ranges. However low molecular weight butyl rubber is less readily available commercially than the high weight variety, and the sealants based in part on low molecular weight butyl are therefore less attractive.
One of the main difficulties in developing an acceptable tire sealant composition is that a single physical property of such a composition can depend on a great variety of chemical variables. Thus in a sealant composition generally comprising a cured, reinforced butyl rubber and a tackifier, a single property such as tensile strength will depend on the fraction of butyl rubber present, the molecular weight and mole percent unsaturation of the butyl rubber, the amount of cross-linking agent used, the amount of reinforcing agent used, and to a certain extent on the tackifiers and on the curing and application techniques employed. Under these circumstances, it is difficult to specify unique ranges for individual chemical variables, since the overall physical properties of interest depend on the combined effect of many variables. For example it has been found that compositions in which the fraction of butyl rubber present lies within a certain range may be formulated having desirable tensile properties. Yet it may well be possible to reproduce such properties outside such range by adjusting the amount of cross-linking agent used and other variables.
In order for a sealant to be practical, it must also be formulated so that it can be easily and practically applied to the tire or article on which it is to be used. One method of applying sealant to the interior of a tire includes the steps of mixing the sealant composition with a curing agent and spraying it into the tire interior as the tire is rotated. Some curing of the sealant occurs as the tire is rotated, resulting in a uniform, seamless coating which resists running and pooling.
Solvents such as toluene may be added to the uncured sealant to lower its viscosity in order to facilitate its application. The use of large quantities of solvents in sealant compositions, however, is undesirable. Many solvents which may be used to reduce the sealant viscosity are highly volatile and flammable and therefore present a health and safety risk to workers processing the tires. The expense of the solvents also adds to the cost of the sealant application process.
The viscosity of the sealant may also be reduced by heating, although such heating increases the rate of curing, and makes application of the sealant more difficult.